High impact polystyrene (HIPS) is an example of graft polymer formation between polystyrene and polybutadiene. A wide variety of peroxy compounds is known from the literature as initiators for the production of styrenic polymers. Commercially available initiators for polymer production may be classified in different chemical groups, which include diacylperoxides, peroxydicarbonates, dialkylperoxides, peroxyesters, peroxyketals, and hydroperoxides. Peroxides and hydroperoxides undergo at least four reactions in the presence of monomers or hydrocarbons with double bonds. These reactions are: 1) chain transfer, 2) addition to monomer, 3) hydrogen abstraction, and 4) re-combination, often called a cage effect.
Hydroperoxides have been shown to undergo induced decomposition reactions, in which a polymer radical (˜˜P*) will react with the initiator as shown below. This reaction is basically a chain transfer reaction and the reaction should be amenable to the well-known chain transfer equations. Radicals obtained from peroxide initiators (RCOO*) can also abstract a hydrogen from the hydroperoxide.RCOO* or ˜˜P*+RCOOH→˜˜PH+ROO*Baysal and Tobolsky (Journal of Polymer Science, Vol. 8, p. 529 et seq., (1952), incorporated by reference herein) investigated the chain transfer of polystyryl radicals to t-butyl hydroperoxide (t-BHP), cumyl hydroperoxide (CHP), benzoyl peroxide (Bz2O2), and azobisisobutyronitrile (AIBN). AIBN and benzoyl peroxide give the classical linear correlations between rate and 1/DP (Degree of Polymerization) indicating no chain transfer to initiators. The hydroperoxides, however, show significant levels of chain transfer.
A. I. Lowell and J. R. Price (Journal of Polymer Science, Vol. 43, p. 1, et seq. (1960), incorporated by reference herein) also showed that polystyryl radicals undergo considerable chain transfer with bis(2,4-dichloro) benzoyl peroxide as compared to dilauroyl peroxide.
The transition metal catalyzed peroxidation of the pendant allylic functionality in ethylene-propylene-diene monomer (EPDM) rubbers with tertiary butyl hydroperoxide results in elastomeric, high polymer peroxides, according to B. Dean in “Graft Copolymers from Peroxidized EPDM Rubber,” Journal of Applied Polymer Science, Vol. 32, pp. 5619–5625 (1986), incorporated by reference herein. The peroxidized EPDM rubbers are useful as free radical initiators for the polymerization and grafting of vinyl monomers in the preparation of comb-type structures. The grafted EPDM rubbers are efficient impact modifiers for thermoplastic resins so long as the polymer grafted onto the EPDM is identical to or is thermodynamically miscible with the composition of the thermoplastic resin.
Cobalt naphthenate and hydroperoxides of methyl ethyl ketone or other hydroperoxides are known to initiate styrene polymerizations at room temperature, and in fact, this reaction is used to form castings.
The ductile properties of HIPS are controlled primarily by the soft rubbery component. Much effort has been expended in developing methods to control the rubber particle size and the rubber morphology of the rubber phase, which is composed of polybutadiene (PB), trapped polystyrene (PS), and PS grafted on PB. The size of rubber particles in HIPS is determined by adjusting the bulk viscosity of the incompatible PS and PB phases, the shear rate, as well as the temperature and other factors. Rubber particle size can also be controlled by using additives, such as styrene-butadiene copolymers.
It would be desirable if methods could be devised or discovered to improve and control the properties of HIPS, including the resultant rubber particle size that occurs during this polymerization.